In
cooperation with the U.S. Air Force, Aeronautical Systems Center, Environmental
Management Directorate, Wright-Patterson Air Force Base, Ohio

Hydrogeology and
Simulation of Ground-Water Flow in the Paluxy Aquifer in the Vicinity of Landfills
1 and 3, U.S. Air Force Plant 4, Fort Worth, Texas

By Eve L. Kuniansky and Stanley T.
Hamrick

U.S. Geological Survey
Water-Resources Investigations Report 98–4023

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Average daily production for White Settlement municipal supply wells completed
in the Paluxy aquifer

6.

Water budget for steady-state simulation, May 1993

7.

Water budget for steady-state simulation with recovery wells

Abstract

Ground-water contamination of the surficial terrace alluvial aquifer has occurred
at U.S. Air Force Plant 4, a government-owned, contractor-operated facility,
northwest of Fort Worth, Texas. A poorly constructed monitoring well, P–22M,
open to the underlying middle zone of the Paluxy aquifer was installed at landfill
3, October 1987, allowing leakage of contaminated ground water to reach the
Paluxy aquifer. This well was plugged and abandoned in November 1995. Additionally,
volatile organic compounds have been detected in fractures in the Goodland-Walnut
confining unit, the hydrogeologic unit separating the terrace alluvial aquifer
from the underlying Paluxy aquifer, beneath the western part of landfill 1.
Volatile organic compounds in concentrations near the analytical detection limit
were detected in the upper Paluxy prior to the drilling of well P–22M.

The ground-water-flow simulation model described in this report was developed
to examine the best logistically feasible location to install recovery wells
to capture the low concentration (less than 100 micrograms per liter) trichloroethylene
plume beneath landfills 1 and 3 (west Paluxy plume). Once the recovery wells
were installed (1996), the simulation model was recalibrated with new data.
This report documents the capture area of the installed recovery wells. Four
geologic units are pertinent to this site-specific model. From oldest to youngest,
these are the Glen Rose Formation, Paluxy Formation, Walnut Formation, and Goodland
Limestone. The Glen Rose Formation is relatively impermeable in the study area
and forms the confining unit underlying the Paluxy Formation. The Paluxy Formation
forms the Paluxy aquifer, which is a public drinking water supply for the City
of White Settlement. The Walnut Formation and Goodland Limestone form the Goodland-Walnut
confining unit overlying the Paluxy aquifer. Near landfill 3, gamma-ray logs
indicate three distinct zones of the Paluxy Formation; upper, middle, and lower.
The formation is about 170-feet thick near landfill 3, and each zone is about
57-feet thick.

Two steady-state simulations using the computer program MODFLOW were analyzed
using the particle-tracking computer program, MODPATH. One simulation is the
calibration simulation using Paluxy aquifer water-level data for May 1993. The
second simulation includes the installed recovery wells. A variably spaced grid
was designed for the model. The smallest grid cells, 25 by 25 feet, are in the
vicinity of landfills 1 and 3. The largest cells, 4,864.5 by 1,441.5 feet, are
at the northwestern corner of the model grid near the Parker-Tarrant County
line. The modeling was accomplished with three layers representing the upper,
middle, and lower zones of the Paluxy aquifer. Particles, which represent contaminant
molecules moving in solution with the ground water, were tracked from well P–22M
and an area below landfill 1, at the top of the upper zone of the Paluxy aquifer,
for 9 years (forward tracking). The forward tracking estimates where contaminants
might move by advection from 1987 to 1996. Analysis of backward tracking from
the new recovery wells indicates that the simulated contributing area to the
recovery wells intercepts the contaminant plume, minimizing off-site migration
of the west Paluxy plume. To determine the effectiveness of the recovery wells,
monitoring wells southeast of Building 14 have been installed (1996–97)
for sampling.